1. Field of the Invention
The present invention relates to an optical guide waveguide element, a manufacturing method for optical waveguide elements, an optical deflection element and an optical switching element, and more particularly to an optical waveguide element capable of being coupled with an optical fiber at high coupling efficiency and a manufacturing method for such optical waveguide elements, an optical deflection element and an optical switching element to which the optical waveguide element according to the invention is applied.
2. Description of the Related Art
Conventionally, glass such as silica, oxide ferroelectrics and electro-optical materials such as LiNbO3, magneto-optical materials such as Y3Ga5O12, polymers such as PMMA, and GaAs-based chemical compound semiconductors are used for planar optical waveguides. Among these materials, oxide ferroelectrics are known to manifest particularly satisfactory acousto-optical and electro-optical effects, but most of the actually produced acousto-optical elements and electro-optical elements use LiNbO3.
There are a wide variety of oxide ferroelectrics including LiNbO3, BaTiO3, PbTiO3, Pb1-xLax(ZryTi1-y)1-x/4O3 (which may be PZT, PLT or PLZT depending on the values of x and y), Pb(Mg⅓Nb⅔)O3, KNbO3, LiTaO3, SrxBa1-xNb2O6, PbxBa1-xNb2O6, Bi4Ti3O12, Pb2KNb5O15, and K3Li2Nb5O15, and many of them are superior in characteristics to LiNbO3. Especially, Pb1-xLax(ZryTi1-y)1-x/4O3 is known as a material having a much higher electro-optical coefficient than LiNbO3. The electro-optical coefficient of LiNbO3 monocrystals is 30.9 pm/V whereas that of PLZT (8/65/35:x=8%, y=65%, 1-y=35%) ceramics is as high as 612 pm/V.
The reason why most of the elements actually produced use LiNbO3 or LiTaO3, in spite of the availability of many ferroelectrics with better characteristics than LiNbO3, is that monocrystal growth technology and waveguide formation technology by Ti diffusion to wafers or proton exchange are well established for LiNbO3 and LiTaO3, while thin films need to be formed by epitaxial growth for other materials than LiNbO3 and LiTaO3, and conventional vapor-phase growth cannot provide thin film optical waveguides of high enough quality for practical use.
The present inventors, with a view to solving this problem, proposed a solid phase epitaxial growth technique capable of providing thin film optical waveguides of high enough quality for practical use (Japanese Published Unexamined Patent Application No. Hei 7-78508), but an epitaxially grown thin film optical guide often has to be thinner than the mode field diameter of the optical fiber on account of the requirement for singleness of the mode or that to reduce the drive voltage, inviting an increased loss in coupling with the optical fiber.
Previously, regarding semiconductor optical waveguides and silica-based waveguides, techniques to provide a flared optical waveguide in the position of connection to the optical fiber to reduce the coupling loss between the optical waveguide and the optical fiber were proposed in Japanese Published Unexamined Patent Application No. Hei 9-61652 and Japanese Published Unexamined Patent Application No. 5-182948 among others.
However, there is no technique available for producing a fine pattern in the epitaxially grown oxide thin film optical waveguide, making it difficult to fabricate the optical waveguide in a flared shape. For LiNbO3 monocrystalline wafers for example, a fabrication method for three-dimensional optical waveguides to which Ti diffusion and proton exchange are described by Nishihara, Haruna and Suhara in a publication on optical integrated circuits by Ohmsha (1993) pp. 195-230, but no method for other elements or exchanging ions is known for other materials, especially Pb1-xLax(ZryTi1-y)1-x/4O3. For silica-based optical waveguides and the like, a method for producing channel optical waveguides by reactive ion etching is disclosed by Kawachi in NTT RandD, 43 (1994)1273 and elsewhere, but it is difficult to accomplish selectively etch a monocrystalline epitaxial ferroelectric thin film optical waveguide without roughing its surface, which would invite scattering loss or damaging the substrate, which is an oxide of the same kind as the thin film optical waveguide. For this reason, no report is found on the successful fabrication of any relatively loss-free channel optical waveguide into an epitaxial ferroelectric thin film optical waveguide. Furthermore, there is another problem that merely flaring an epitaxially grown oxide thin film optical waveguide can hardly prevent the waveguide mode from becoming multi-mode.
The present invention, therefore, provides an optical waveguide element which can be coupled with an optical fiber at high coupling efficiency. The invention also provides a manufacturing method for optical waveguide elements which can be coupled with optical fibers at high coupling efficiency. The invention further provides an optical switching element and an optical deflection element enabled to achieve coupling with an optical fiber at high coupling efficiency by applying the optical waveguide element according to the invention.
In order to achieve one of these intentions, an optical waveguide element according to an aspect of the invention is provided with an optical waveguide layer having an optical waveguide, and a cladding layer which is provided over at least one of the incidence end and the emission end of the optical waveguide on the surface of the optical waveguide layer, has a lower refractive index than that of the optical waveguide layer, and gradually increases in thickness towards the end(s) in a flared shape.
According to another aspect of the invention, there is provided an optical waveguide element manufacturing method for manufacturing optical waveguide elements by forming, over the surface of an optical waveguide layer provided with an optical waveguide, an amorphous thin film whose refractive index is smaller than that of the optical waveguide layer, shaping the amorphous thin film over at least one of the incidence end and the emission end of the optical waveguide to leave a flared part whose thickness increases towards the end(s), and forming the shaped amorphous thin film into a cladding layer by solid phase epitaxial growth.
Another manufacturing method for optical waveguide elements according to another aspect of the invention includes an optical waveguide formation step to shape an amorphous thin film formed over the surface of a monocrystalline substrate into a prescribed channel pattern, form a buffer layer by subjecting the amorphous thin film so shaped to solid phase epitaxial growth, and form a channel optical waveguide by solid phase epitaxial growth of an optical waveguide layer over the buffer layer, and a cladding layer formation step to form an amorphous thin film whose refractive index is smaller than that of the optical waveguide layer over the surface of the optical waveguide layer provided with the optical waveguide, shape the amorphous thin film over at least one of the incidence end and the emission end of the optical waveguide to leave a flared part whose thickness increases towards the end(s), and form a cladding layer by subjecting the amorphous thin film so shaped to solid phase epitaxial growth by heating.
An optical deflection element includes an optical waveguide layer having an epitaxial or single-oriented electro-optical effect, provided over an electroconductive or semi-electroconductive monocrystalline substrate to serve as a lower electrode or over a substrate over the surface of which is formed an electroconductive or semi-electroconductive monocrystalline substrate to serve as a lower electrode; a light beam controlling electrode which is arranged over the optical waveguide layer and forms, between the optical waveguide layer and the monocrystalline substrate or the monocrystalline thin film, a region having a refractive index varying with the voltage applied and deflecting a light beam which propagates through the optical waveguide layer according to the voltage applied; and a cladding layer which is formed over at least one of the incidence end and the emission end of the optical waveguide over the surface of the optical waveguide layer and has a smaller refractive index than that of the optical waveguide layer, and whose thickness gradually increases towards the end(s) in a flared shape.
An optical switching element according to another aspect of the invention includes an optical waveguide layer having an epitaxial or single-oriented electro-optical effect, provided over an electroconductive or semi-electroconductive monocrystalline substrate to serve as a lower electrode or over a substrate over the surface of which is formed an electroconductive or semi-electroconductive monocrystalline substrate to serve as a lower electrode; an optical waveguide formed in the optical waveguide and having at least one branched part; upper electrodes one of which is provided for each branch of the branched part; and a cladding layer which is formed over at least one of the incidence end and the emission end of the optical waveguide over the surface of the optical waveguide layer and has a smaller refractive index than that of the optical waveguide layer, and whose thickness gradually increases towards the end(s) in a flared shape.
As described above, the optical waveguide element according to one aspect of the invention has, over at least one of the incidence end and the emission end of the optical waveguide over the surface of the optical waveguide layer, a cladding layer which has a smaller refractive index than that of the optical waveguide layer and whose thickness gradually increases towards the end(s) in a flared shape.
Thus, by providing a cladding layer whose refractive index is smaller than that of the optical waveguide layer over the optical waveguide, it is made possible to expand the diameter of the mode field in the optical waveguide and thereby substantially reduce the coupling loss between the optical fiber and the optical waveguide element. Furthermore, the shape of the cladding layer whose thickness gradually increases towards the end(s) over at least one of the incidence end and the emission end of the optical waveguide can also help reduce the light propagation loss in the optical waveguide element to a negligible level.
The manufacturing method for optical waveguide elements according to another aspect of the invention, makes it easier to shape the cladding layer and enables optical waveguide elements to be produced more precisely than where a thin film obtained by solid phase epitaxial growth is shaped, because first an amorphous thin film having a smaller refractive index than that of the optical waveguide layer is formed over the surface of the optical waveguide layer provided with an optical waveguide, then this amorphous thin film is so shaped as to leave a flared part whose thickness increases towards the end(s) over at least one of the incidence end and the emission end of the optical waveguide, and the shaped amorphous thin film is further formed into a flared shape through solid phase epitaxial growth by heating.
Another manufacturing method for optical waveguide elements according to still another aspect of the invention, makes possible, in addition to the formation of the cladding layer in the same manner as the foregoing manufacturing method for optical waveguide elements, shaping of an amorphous thin film formed over the surface of the monocrystalline substrate into a prescribed channel pattern, formation of the shaped amorphous thin film into a buffer layer by solid phase epitaxial growth, and formation of a channel optical waveguide through solid phase epitaxial growth of an optical waveguide layer over this buffer layer, resulting in an optical waveguide element having a fine pattern of the channel optical waveguide, whose side walls and surface are smooth and which is hardly susceptible to scattering loss.